1. Field of the Invention
This invention relates to a magnetism-erasing circuit for use in cathode ray tubes, and more particularly to a magnetism-erasing circuit for erasing the magnetism from metal structures of cathode ray tubes or from the shadow masks of the cathode ray tubes, whereby mislanding of the electron beams on the cathode ray tubes or the shadow masks of the cathode ray tubes is prevented.
2. Description of the Conventional Art
Generally, when power is initially turned off and then re-applied, with a beam of electrons being projected on cathode ray tubes of televisions or monitors, the beam of electrons emitted by electron guns cannot be focused to a predetermined area on the fluorescent screens since the metal structures in the cathode ray tubes are magnetized. That is, the beam of electrons is mislanded on the fluorescent screen in the cathode ray tubes.
Magnetism-erasing circuits are generally provided to prevent this mislanding. For this purpose, predetermined magnetism-erasing currents flow through magnetism-erasing coils which are wound around the panels of the cathode ray tubes for generating magnetic fields, whereby the magnetism of the metal structures are erased.
FIG. 1 shows a conventional magnetism-erasing circuit wherein alternating current is used. In conventional magnetism-erasing circuits, when an A-C supply is initially applied to the conventional magnetism-erasing circuit, temporary large currents will flow through a magnetism-erasing coil 70 via a posistor resistance R2 and less currents will flow through posistor resistance R1 (R1&gt;R2). Since the resistances R1 and R2 are posistors, their resistance values are increased suddenly as current flows. After a few seconds, the resistance values become hundreds of K.OMEGA.. Accordingly, currents of waveforms as shown in FIG. 2 will flow through the magnetism-erasing coil 70, causing the magnetism to be erased from the metal structures.
Conventional magnetism-erasing circuits have a drawback that only an A-C supply may be used. Accordingly, the conventional art magnetism-erasing circuit cannot be used in portable televisions or instruments in which an A-C supply is not used or is unavailable.